1. Field
The present invention relates to a set of elevators with common control system and to a method for controlling a set of elevators with common control system.
2. Description of Related Art
One of the objectives in elevator development work is to achieve an efficient and economical utilization of building space. When a plurality of elevators are installed in a building, quite often so as to form one or more elevator groups, efficiency of space utilization becomes more important. Elevators without machine room have made it possible to achieve significant space savings in buildings. Likewise, space savings have been achieved in the elevator groups themselves by providing common machine rooms for the elevators in the elevator group to house the machinery of two or more elevators.
By using an elevator construction without counterweight, significant savings of building space are achieved as there is no need to reserve any space in the elevator shaft for the passage of a counterweight. A prior-art solution for a traction sheave elevator without counterweight is disclosed in specification U.S. Pat. No. 5,788,018, in which the elevator car is suspended with a 1:1 suspension ratio and which uses a separate controllable tensioning device for the tensioning of the rope loop. A recent traction sheave elevator solution without counterweight is described in specification WO2004041704, which discloses an elevator in which the motion of the elevator car is based on friction with elevator hoisting ropes driven by a traction sheave. This solution is primarily targeted for low buildings and/or elevators with a low hoisting height. The specification provides solutions that are primarily applicable for use in relatively low buildings, although the ideas are also applicable in the case of larger hoisting heights. Large hoisting heights and higher speeds involve new problems to be solved.
In respect of energy consumption, an elevator without counterweight differs from an elevator provided with a counterweight. Especially in the case of larger hoisting heights, controlling the energy consumption of the elevator is important. The reason is that the movement of the elevator requires a large amount of power, which is dominantly directly dependent on the speed of the elevator. While in an elevator with counterweight the counterweight functions as an energy storage, receiving and delivering the potential energy resulting from the up and down movement of the elevator car on its track, an elevator without counterweight has no such storage of mechanical energy, which leads to a need to design the elevator hoisting machine and the electric drive supplying the hoisting machine for very high power ratings, although the masses to be moved in an elevator without counterweight are smaller than in an elevator with counterweight and thus also the required acceleration power is lower than in an elevator with counterweight. Energy consumption in itself is only one factor to be considered when estimating the suitability of an elevator for a building. Providing a building with several elevators increases the power requirement, which leads to a need for a larger power supply. The size of the power supply, i.e. the size of the so-called main fuse of the power supply, is a significant cost factor in the building, and in some cases the size of the power supply may be a limiting factor; for example, in existing buildings the supply cable of the building or the transformer supplying electricity for use in the building imposes a maximum limit on the electric power available. Within the building there may also be corresponding limitations. Consequently, it is advantageous to design the elevator system in respect of power supply in a manner such that it will require a low supply power.